The invention is directed to a wavelength-division multiplex system and to a method for measuring the individual crosstalk of a specific, of a plurality or of all channels on a payload channel in optical transmission systems, particularly in wavelength-division multiplex systems, comprising n WDM channels, an input-side multiplexer that combines the n WDM channels supplied to it with different frequencies, a following transmission link and an output-side demultiplexer that respectively divides an optical signal incoming on a fiber onto n WDM channels.
In optical transmission systems, the technique of wavelength-division multiplex (WDM) is a frequently utilized method for utilizing the greatest bandwidth of the optical fiber. Within the framework of network monitoring of such systems, it is often desirable to know the crosstalk behavior of all channels on a payload channel being observed. The knowledge of the crosstalk of only one defined channel on the payload channel being observed or the statement about the proportion with which the individual channels participate in the overall crosstalk can also be significant.
For work in the laboratory, there is the solution that the optical spectrum is registered with an extremely great precision. Lines appear in the spectrum at the locations of the crosstalking channels, but these can only be resolved with great outlay. The crosstalk can be identified by measuring these lines and the signal power of the payload channel. This solution is so involved that it can only be employed in the laboratory. It is too complicated and too expensive or, respectively, not economically feasible for an employment in general operations.
It is therefore an object of the invention to develop a method for measuring the individual crosstalk of a specific, or a plurality or of all channels on a payload channel in wavelength-division multiplex systems that is significantly simplified.
Moreover, a wavelength-division multiplex system for application of the inventive method is presented.
The object of developing a method is achieved by the features of the first method claim; the object of developing a wavelength-division multiplex system is achieved by the features of the first apparatus claim.
The inventors have recognized that, given a frequency impressed onto an optical payload signal, the part of the photocurrent of the payload signal this is picked up by a photodiode and is synchronous with the impressed frequency derives as
IPhxe2x80x94synch,channel=mxc2x7RPhxc2x7TDEMUXxc2x7PSignalxe2x80x94opt,channelxe2x80x83xe2x80x83(1)
with the transmission of the demultiplexer TDEMUX, the responsitivity of the photodiode RPh, the modulation factor m and the optical signal power of the WDM channel PSignalxe2x80x94opt,channel.
The synchronous parts of each individual crosstalking signal (n) are calculated from
IPhxe2x80x94synch,frequency(n)=m(n)xc2x7RPhxc2x7TDEMUXxc2x7PSignalxe2x80x94opt,frequency(n)xe2x80x83xe2x80x83(2)
with the corresponding modulation factor m(n) and the optical signal power of the corresponding WDM channel PSignalxe2x80x94opt,frequency(n).
The non-synchronous part of the photocurrent is composed of the non-modulated signal levels of the payload channel (1-m(n)) PSignalxe2x80x94opt,frequency, the optical noise Pnoisexe2x80x94opt and the photodiode dark current IDxe2x80x94Ph. The non-synchronous part derives as                                                         I              Ph_nonsynch                        =                                                            R                  Ph                                ·                                  T                  DEMUX                                ·                                  [                                                            (                                                                        ∑                                                      n                            =                            1                                                                                M                            -                            1                                                                          ⁢                                                  xe2x80x83                                                ⁢                                                                              (                                                          1                              -                              m                                                        )                                                                                (                            n                            )                                                                                              )                                        ·                                          P                                              Signal_opt                        ,                        frequency                                                                    (                        n                        )                                                                              )                                            +                                                (                                      1                    -                    m                                    )                                ⁢                                  P                                      Signal_opt                    ,                    channel                                                              +                                                NB                  DEMUX                                ·                                  P                  Noise_opt                                                              ]                +                  I          D_ph                                    (        3        )            
with the effective noise bandwidth of the optical demultiplexer NBDEMUX.
This non-synchronous part can also be utilized for determining the optical signal-to-noise ratio (OSNR).
The optical signal power of the WDM payload channel derives from (1) as                               P                      Signal_opt            ,            channel                          =                              I                          Ph_synch              ,              channel                                            m            ·                          R              Ph                        ·                          T              DEMUX                                                          (        4        )            
The optical signal power of the individual crosstalking channels derives from (2) as                                           P                          Signal_opt              ,              frequency                                            (            n            )                          =                              I                          Ph_synch              ,                              frequency                                  (                  n                  )                                                                                        m                              (                n                )                                      ·                          R              Ph                        ·                          T              DEMUX                                                          (        5        )            
The crosstalk (CT) of an arbitrary channel on the payload channel is defined from                               CT                                    Frequency                              (                n                )                                      ↔            channel                          =                  10          ·                      log            ⁡                          (                                                P                                      Signal_opt                    ,                                          frequency                                              (                        n                        )                                                                                                              P                                      Signal_opt                    ,                    channel                                                              )                                                          (        6        )            
Consequently following with (4) and (5) is                               CT                                    Frequency                              (                n                )                                      ↔            Channel                          =                  10          ·                      log            ⁡                          (                                                                    I                                          Ph_synch                      ,                                              frequency                                                  (                          n                          )                                                                                                                          I                                          Ph_synch                      ,                      channel                                                                      ·                                  m                                      m                                          (                      n                      )                                                                                  )                                                          (        7        )            
It is meaningful, although not necessary, to employ the same modulation factor for all channels. Equation (7) is then simplified into                               CT                                    Frequency                              (                n                )                                      ↔            Channel                          =                  10          ·                      log            ⁡                          (                                                I                                      Ph_synch                    ,                                          frequency                                              (                        n                        )                                                                                                              I                                      Ph_synch                    ,                    channel                                                              )                                                          (        8        )            
Only the ratio of the synchronous currents thus need be considered for the calculation of the crosstalk. The crosstalk calculated by Equation (8) thus reproduces the influence of a defined channel on the payload channel.
When all influencing channels are to be taken into consideration, then the following is valid for different modulation factors and M crosstalking channels                               CT                      Frequency            ↔            Channel                          =                  10          ·                      log            (                                          (                                                      ∑                                          n                      =                      1                                                              M                      -                      1                                                        ⁢                                      xe2x80x83                                    ⁢                                                                                    I                                                  Ph_synch                          ,                                                      frequency                                                          (                              n                              )                                                                                                                                                  m                                                  (                          n                          )                                                                                      ·                    m                                                  )                                            I                                  Ph_synch                  ,                  channel                                                      )                                              (        9        )            
The following is valid for identical modulation factors                               CT                      Frequency            ↔            Channel                          =                  10          ·                      log            (                                                            ∑                                      n                    =                    1                                                        M                    -                    1                                                  ⁢                                  I                                      Ph_synch                    ,                                          frequency                                              (                        n                        )                                                                                                                        I                                  Ph_synch                  ,                  channel                                                      )                                              (        10        )            
Since no component-dependent quantities are utilized for the calculation, the determination of the crosstalk is independent of the generation of the identification frequency as well as of the components needed for the evaluation.
In conformity with the above perceptions, the inventors, according to claim 1, propose that a method for measuring the individual crosstalk of a specific, of a plurality or of all channels on a payload channel in optical transmission systems, particularly in wavelength-division multiplex systems, comprising n WDM channels, an input-side multiplexer that combines the n WDM channels supplied to it with different frequencies, a following transmission link and an output-side demultiplexer that respectively divides an optical signal incoming on a fiber onto n WDM channels be improved to the effect that n identification frequencies f(n) are modulated onto a payload signal with a modulation factor m(n) and the current crosstalk is calculated.
The current crosstalk can be calculated by coupling a part of the optical power out of the transmission path.
The current crosstalk can also be calculated at the end of the transmission path.
For determining the crosstalk, a low-frequency identification frequency f(n) with the modulation index m(n) can be additionally modulated onto the high-frequency payload signal. This identification frequency is unambiguously measurably lower in frequency than the payload signal, namely such that it does not influence the behavior of other components in the transmission path such as, for example, fiber amplifiers (EDFAsxe2x80x94erbium doped fiber amplifiers).
The identification frequency differs for each WDM channel in the system.
These n different identification frequencies can, for example, be generated by variable optical attenuator.
Different identification frequencies can also be generated by low-frequency, direct modulation of the lasers or, respectively, of the transmission diode. Further, Mach-Zehnder or some other modulators can also be employed for impressing the identification frequencies.
It can also be advantageous to derive the identification frequency from the high-precision system clock of the transmission system that is also available to other network elements. Further, the identification frequency can be derived from the data rate, for example given SDH signals (SDH=simultaneous data hierarchy), so that a phase-locked reference can be made available for the lock-in amplifiers.
The various optical signals are superimposed on a fiber by the multiplexer.
In order to detect the current crosstalk at a specific location, a part of the optical power is coupled out of the transmission path with the assistance of a tap coupler.
Subsequently, the individual WDM channels can in turn be spatially separated from one another by a demultiplexer and the signals can be detected by photodiodes thereafter. The detection of n signals after the demultiplexer can ensue with the assistance of at least one through n photodiodes. When the detection ensues with a photodiode, the n signals can be redirected onto this photodiode with the assistance of a switch following the demultiplexer.
When the crosstalk is detected at the end of the transmission path, the existing demultiplexer as well as the existing receivers such as, for example, n photodiodes can be used. Coupling a part of the signal out is not required here.
The photocurrent supplied by the photodiode is composed of a part of the actual payload signal IPhxe2x80x94synch,channel that is synchronous with the frequency modulated on, of a synchronous part of each individual crosstalking signal IPhxe2x80x94synch,frequency(n), as well as of a non-synchronous part IPhxe2x80x94nonsynch of all optical signals present on the channel.
The synchronous parts can be identified independently of one another by a phase-synchronous detector (what is referred to as a lock-in amplifier). The crosstalk of a specific channel, of a plurality of channels or of all channels on a payload channel can be calculated therefrom. The non-synchronous part can also be defined by measuring the overall power as well as the synchronous parts.
The crosstalk can be calculated from the relationship of the synchronous currents.
The crosstalk of an arbitrary channel on the payload channel can be determined via Equation (7).
The crosstalk of all influencing channels on the payload channel can be determined via Equation (9).
No component-dependent quantities are utilized for the evaluation of the crosstalk.
Particularly for utilizing the aforementioned method, the inventors also propose that a wavelength-division multiplex system comprising n WDM channels, an input-side multiplexer that combines the n WDM channels supplied to it with different frequencies, a following transmission link and an output-side demultiplexer that respectively divides an optical signal incoming on a fiber onto n WDM channels be improved to the effect that means for determining the crosstalk are provided.
Advantageously, the means for determining the crosstalk can be fashioned such that the crosstalk is determined with the assistance of n identification frequencies f(n) with a modulation factor m(n) that are modulated onto a payload signal.
The means for determining the crosstalk can be fashioned such that the crosstalk is determined by coupling a part of the optical power out from the transmission path.
Further, the means can be fashioned such that the crosstalk is determined at the end of the transmission path.
The n identification frequencies f(n) modulated onto the payload signal here can be unambiguously measurably lower in frequency than the payload signal, namely such that they do not influence the behavior of other components in the transmission path such as, for example, fiber amplifiers.
The n identification frequencies f(n) can thereby differ for each WDM channel in the system.
Variable attenuators can be used for generating n different identification frequencies f(n).
Lasers and/or transmission diodes that generate n different identification frequencies by low-frequency, direct modulation can also be employed. Further, Mach-Zehnder modulators as well as differently configured modulators can also be employed for impressing the n identification frequencies.
A tap coupler is present for coupling a part of the optical power out of the transmission path. The current crosstalk in the system can be acquired at this location.
At least one through n photodiodes can be present for the detection of n signals following the demultiplexer.
When the n signals are detected with a photodiode, a switch can also be provided that redirects n signals onto a photodiode following the demultiplexer.
Advantageously, n phase-synchronous detectors (lock-in amplifiers) can be present for the independent determination of the synchronous part of the payload signal and of the synchronous part of the crosstalking signal.
It is self-evident that the aforementioned features and the features of the invention yet to be explained below can be employed not only in the respectively indicated combination but also in other combinations or standing alone without leaving the scope of the invention.
Further features of the invention derive from the subclaims and from the following description of an exemplary embodiment with reference to the drawing.